A plug body for sealing the opening section of the vial container for containing liquid medicinal chemicals is required to have a number of quality performances, e.g., a good capping property, a high sealing ability, a high gas barrier property, a high chemical resistance, a high needlestick resistance and a low reactive property. As a plug body which satisfies these required properties, so called rubber plug has been generally used because of the excellent elasticity.
Among this type of rubber plugs for vial containers, a rubber plug for a vial container which contains medicinal chemicals needs to have quality properties which comply with a test method for an infusion rubber plug which is described in the 16th revised Japanese Pharmacopoeia. In particular, such a rubber plug needs to pass an eluted substance test for one hour at 121° C. using a high pressure steam sterilizer which requires a high heat resistance.
As a material for a rubber plug for a vial container which meets the requirement, synthetic rubber like butyl rubber or isoprene rubber, styrene thermoplastic elastomer like SEBS and thermoplastic elastomer which comprises polyisobutylene or polybutadiene as a major constituent have been conventionally utilized (see Patent Document 1).
In general, a rubber plug for a vial container is formed so as to have a thick discoid cap portion and a thick cylindrical leg portion. The cylindrical leg portion has a diameter which is smaller than that of the cap portion and protrudes from the bottom surface of the cap portion. The lower surface of the flange of the discoid cap portion can closely contact with the end surface of the opening section of the vial container by driving the cylindrical leg portion into the inner periphery of the opening section of the vial container.
This type of rubber plugs for vial containers is generally known as rubber plugs having different materials between the discoid cap portion and the cylindrical leg portion as described below by using a two step molding method including a first step for press molding a cylindrical leg portion alone and a second step for press molding a discoid cap portion integral with the cylindrical leg portion after the first step (see Patent Documents 2 and 3).
That is to say, the leg portion is formed by silicone rubber mixed with elastomer which does not become hard even at extremely-low temperatures, and the cap portion is formed by butyl rubber having a high gas barrier performance and a good coring property.
Additionally, as shown in FIG. 8, it is known to form a convex section 1B′ having a trapezoidal shape in cross section at the connected portion between a cylindrical leg portion 1B and a discoid cap portion 1A, and form a concave portion 1A′ at the discoid cap portion 1A to fit to the convex portion 1B.